1. Field of the Invention
The invention relates to a power coupler with an adjustable coupling factor for accelerator cavities, in particular superconducting accelerator cavities.
2. Description of the Related Art
High-frequency resonators or accelerator cavities of beam tubes which are used for particle acceleration and/or applications related thereto in accelerator installations include one or more different high-power coupling devices, among other components. Such high-power coupling devices couple the power (HF-power up to 200 kW) to accelerate the particle beam in high-frequency resonators or accelerator cavities of the beam tubes. Usually, such accelerator cavities are operated at resonant frequencies ranging from about 40 MHz to about 5 GHz. Generally speaking, the resonant frequencies of accelerator cavities lie in the high-frequency (HF) domain, and particularly in the radio-frequency (RF) range.
High-power coupling devices are in the form of rod couplers or loop couplers. A rod coupler usually includes a coaxial waveguide connected to the accelerator cavity to be powered or to the beam tube in the vicinity of the cavity; the coaxial waveguide has a tube-shaped outer conductor and a rod-shaped inner conductor which is arranged in the outer conductor and serves as an antenna to transfer HF-power to the cavity. Preferably, rod couplers are used in conjunction with accelerator cavities which are operated at resonant frequencies ranging below about 1 GHz, in particular at several hundred MHz.
Due to the rigid mechanical construction of such couplers, the strength of the coupling, which is expressed as the so-called coupling factor, is fixed and cannot readily be adjusted in dependence of the operational requirements.
In accelerator applications, a typical mode of operation of an accelerating cavity is to sustain an electromagnetic field, oscillating with the resonance frequency of the cavity; the amplitude is predetermined to correspond to the amount of acceleration of particles in a beam passing through. The amplitude of the field is kept constant by providing a high-frequency signal having sufficient power, and coupling an effective fraction of the signal into the cavity by means of a suitable power coupling device. In the absence of a particle beam, the HF-power coupled into the cavity matches the amount of that dissipated in the cavity; for a typical superconducting cavity, this may be approximately 10 W. However, an accelerated beam of particles passing through the cavity increases the HF-power afforded to sustain the electromagnetic field at its predetermined amplitude, as the acceleration process affords a power transfer from the cavity to the beam.
In typical applications, the HF-power transferred to a beam of particles may well amount to several hundred kilowatts. Since HF-power sources capable of varying their power output between about 10 W and about 200 kW are not readily available, an accelerating cavity is usually powered by a HF-power source delivering a signal of constant and sufficiently large amplitude which is coupled to the cavity by means of a power coupling device having a coupling factor adjusted so that the desired amount of power is transferred into the cavity, according to the operating conditions, including the intensity of the beam.
Until now this problem has been solved by the use of different couplers, each adapted to a different purpose and each being selectively connected to the accelerator cavity, according to the specific operational requirements.
However, that method is not flexible and it necessitates additional assembly procedures when the operating conditions change.
German patent specification DE 32 08 655 C2 discloses a power coupling device for a superconducting accelerator cavity with an adjustable coupling factor. That device comprises a coaxial waveguide with an outer conductor and a rod-shaped inner conductor connecting the cavity to a rectangular waveguide leading to a HF-power source. The outer conductor is rigidly fixed between the rectangular waveguide and the cavity, and the inner conductor projects from the coaxial waveguide through the rectangular waveguide to an external drive, and is movable relative to the outer conductor, so as to vary the coupling factor by varying the position of a tip of the inner conductor in the vicinity of the cavity. The rectangular waveguide has a first wall, where the outer conductor terminates leaving an opening into the coaxial waveguide, and a second wall opposite the first wall with another opening through which the inner conductor projects. However, the inner conductor has to be contacted to the second wall by a connection which should be an ideal electric short. According to that patent specification, such a short is approximated by a .lambda./4-transformer, which includes a number of conductive tubes of suitable length (approximately .lambda./4) and differing diameter arranged concentrically with the inner conductor and electrically contacted to the second wall. By dimensioning the transformers accordingly, a relatively small electric impedance close to a short circuit may be attained at the gap between the inner conductor and the second wall. However, the small electric impedance is still dependent on the geometrical configuration of the external drive for the inner conductor which may include cavities with dimensions and, accordingly, resonances which vary as the inner conductor is moved. This entails fluctuations of the impedance at the gap and, of course, may considerably limit the applicability of that power coupling device.
The afore-mentioned problem of electrically connecting the inner conductor of a coaxial waveguide to a wall of a tubular waveguide, especially a rectangular waveguide, might in some circumstances be solved by furnishing sliding contacts directly connecting the inner conductor to the wall; however, in conventional accelerator applications this solution is excluded because of the high electrical currents between the wall and the inner conductor. The currents correspond to the high HF-powers (up to 1 MW) which have to be handled; damage to the sliding contacts would be likely.
A superconducting single cell cavity for pion-beam compression is used at the Los Alamos Laboratory. The cavity operates at a resonant frequency near 400 MHz.
Since the beam load on the cavity is negligible in that particular application, the total quality factor Q of the cavity is essentially determined by the coupler, since the quality factor of a superconducting cavity without any coupling is generally extremely high and no effective load is present from a beam passing through the cavity, which in turn further impairs the quality factor Q. A compromise may be found between a high Q which is desirable for a low level of HF-power needed and a low Q which eases the frequency control of the HF-power source, as a rather low Q corresponds to a fairly high bandwidth of the cavity and consequently may allow a reduction of precision requirements for the frequency control. Besides, a sufficiently low Q assures that the impedance of the cavity is kept almost constant, if frequency variations are kept within reasonable and well achievable limits; accordingly, it is possible to avoid great mismatches which might adversely affect the HF-power source.
In such a situation a need has been found for a simple and reliable power coupler with adjustable coupling, in order to obtain flexibility in view of possible variation in operational conditions as well as to explore the boundaries of the above-mentioned range of Q.